A crescendo in the inner structure of snorts: a reflection of increasing arousal in rock hyrax songs?

Acoustic and rhythmic temporal patterns may have important roles both in animal vocalizations and in human music. Male rock hyraxes, Procavia capensis, advertise their quality and emotional state by singing a long complex song, comprising repetitive elements. Research has revealed that the temporal structure of hyrax songs is similar to a musical crescendo, as it includes a gradual increase in amplitude, along with several additional acoustic and structural parameters, throughout the song. In hyrax songs, the snort is a relatively rare, low and harsh vocal element, which increases in number as the song progresses. We have previously shown that mean snort harshness is positively linked to both the singer's social status and testosterone levels, possibly reflecting inner arousal. Here, we measured the changes in snort harshness and length during natural hyrax songs. Our results indicate that in mature males, harshness, quantified by shimmer and jitter, increases as the song progresses. We suggest that increasing snort harshness may either (1) indicate the progression of inner excitement or aggression throughout the song or (2) that the harshness may reflect the difficulty of vocal control following increasing effort or the physical fatigue of the vocal cords. In western culture music, such progression is used to increase listener tension and thus retain attention, preventing possible habituation. Thus, understanding the development and progression of challenging elements within animal communication systems may reveal a key fundamental root of modern musical structure, and shed light on the evolutionary basis of musical perception.